Mississauga’s climate is considered to be moderate,[26] and is located in plant hardiness zone 6b.[27] Under the Köppen climate classification, Mississauga has a humid continental climate (Dfa/Dfb).[28] Summers can bring periods of high temperatures accompanied with high humidity.[26] While the average daily high temperature in July and August is 27 °C (80.6 °F), temperatures can rise above 32 °C (89.6 °F). In an average summer, there are an average of 15.8 days where the temperature rises above 30 °C (86.0 °F).[29] Winters can be cold with temperatures that are frequently below freezing.[26] In January and February, the mean temperature is −6 °C (21.2 °F) although it is common for temperatures to fall to −15 °C (5.0 °F).[26] In an average winter, there are an average of 3.9 days where the temperature falls below −20 °C (−4.0 °F).[29] Occasionally, there can be brief periods of warmer weather during the winter season.[26] Compared to the rest of Canada and Ontario, the amount of snowfall received during the season is relatively low.[26] On average, Mississauga receives 108.5 centimetres (42.7 in) of snow per year and there are 44.4 days with measurable snowfall.[29] The climate of Mississauga is officially represented by Pearson International Airport but because of its topography and large surface area conditions can differ depending on location: fog tends to be more common along the lakeshore and in the Credit River Valley at certain times of year, particularly during the spring and autumn.

During snowfalls when temperatures hover close to freezing, northern parts of the city, such as around Derry Road, including Pearson Airport away from warmer Lake Ontario usually get more snow that sticks to the ground because of the lower temperatures. The reverse occurs when a strong storm approaches from the south kicking up lake effect snow, bringing higher snowfall totals to south Mississauga. The city usually experiences at least 6 months of snow free weather however there is the odd occurrence where snow does fall either in October or May, none which sticks to the ground. The Port Credit and Lakeview areas have a micro-climate more affected by the proximity of the open lake, warming winter temperatures as a result but it can be sharply cooler on spring and summer afternoons, this can also be the case in Clarkson, but with much less consistency.

Most thunderstorms are not severe but can occasionally bring violent winds. The last known tornado to cause significant damage touched down on 7 July 1985, when an F1 rated tornado struck an industrial park in the Meadowvale area (Argentia Road), heavily damaging some buildings and some parked tractor trailers. A relatively strong tornado tore a path across Mississauga (then part of Toronto Township) on 24 June 1923, cutting a swath from present-day Meadowvale to near Cooksville, killing 4 people and causing massive property damage in a time when most of Mississauga was still rural farmland dotted with fruit orchards.[30][31][32]